Sinus bradycardia: endurance training can reduce the heart rate at rest. Some excellent endurance athletes' heart rate can be as low as 40-60 beats/min when they are quiet, which is called sinus bradycardia. It is the result of strengthening vagus nerve and weakening sympathetic nerve, which control heart activity. Sinus bradycardia is reversible, and the heart rate of elite athletes will return to normal after stopping training.

Exercise-induced cardiac enlargement: Cardiac enlargement is a good adaptation of athletes to long-term exercise load. The enlargement of athletes' heart, which is mainly based on strength exercise, is mainly caused by the thickening of ventricular wall, and their endurance is ventricular cavity.

Improvement of cardiovascular function: the increase of stroke output is the adaptive response of the heart to exercise training. Exercise training can improve the regulation function. When some trainers do quantitative work, the cardiovascular function is mobilized quickly, with great potential and rapid recovery.

Physiology has returned to books, only remembering that it is related to stroke output.

Because the oxygen demand is fixed, the stroke output increases and the heart rate naturally decreases. Then the significance of physiology is obvious. When the amount of exercise increases, the heart rate will be lower than before, and it is dangerous for people to exceed the maximum heart rate (220- age), so the stroke output ensures that the heart rate is within a healthy range. In addition, long-term fitness will increase blood oxygen content because of some mechanism I have read about in the book, and the benefits are the same as above.